Three major projects are proposed: (1) Continue work on the role of T-antigen in cells lytically infected by SV40, in particular role in feedback regulation of synthesis by studying early RNA levels in ts mutants; identification of structures to which T-antigen is bound in infected cells, using electron microscopy. (2) Continue work on characterization and properties of an RNA virus of broad host range found in several lines of hamster cells. (3) Continue characterization of hamster cells that greatly overproduce aspartate transcarbamylase and 2 other early pyrimidine enzymes, selected with N-(phosphonacetyl)-L-aspartate. Purify the proteins, isolate mRNAs, determine dominance of the resistant-phenotype. BIBLIOGRAPHIC REFERENCES: Reed, S.I., Ferguson, J., Davis, R.W. and Stark, G.R., T Antigen Binds to Simian Virus 40 DNA at the Origin of Replication, Proc. Nat. Sci., 72 1605 (1975). Noyes, B.E. and Stark, G.R., Nucleic Acid Hybridization Using DNA Covalently Coupled to Cellulose, Cell, 5, 301(1975).